Carburizing steels



Patented Aug. 7, 1945 CARBUBIZING STEELE Frederick It. Bonte, Canton, Ohio, assignor to The Timken Roller a corporation of Ohio No Drawing. Application May 20, 1043. I Serial No. 487.812

8 Claiml. (Cl. 75-123) This invention relates to carhurized steel articles and steels for making them.

Carburized articles have been made extensively from plain carbon steels. However, the core properties of such articles are inadequate for many purposes and for that reason alloy steels have been used widely for the manufacture of such articles, especially where they are subjected to high stresses, for example bearings and gears. The belief has been that nickel is advantageous in such steels, not only to develop satisfactory core properties but also to produce cases that are satisfactorily resistant to spalling and related phenomena, so that the bulk oi carburized articles for resisting stresses have been produced from nickel-containing steels. Chromium and, in many instances, molybdenum have been used also in such carburizing steels. Examples of alloy steels used for carburizing are S. A. E. 2115, 3220, 4615, 4620, and 5120.

Although there are ample deposits of molybdenum ore in this country, nickel and chromium are among the so-called strategic metals the supplies of which are inadequate to meet both the demands of the armed forces and also the industrial demands for steels containing them. The necessity for substituting plain carbon steel for the foregoing and other carburizing steels would work a hardship upon various industries because of the inadequacy of carburized plain carbon steel articlesfor some purposes. However, fully satisfactory substitutes for the alloy steels used previously for carburizing purposes have not been available. Such substitute steels should not only provide adequate mechanical properties, both in the core and in the case, but also should be capable of being carburized in the same manner as the S. A. E. alloy steels because the treatments used for the latter have been carefully standardized by the various producers, and the necessity of shifting to different operating conditions would be objectionable.

It is among the objects of this invention to supply carburizing steels which are of simple composition, do not require chromium or nickel, may be carburized by existing practices to provide cases of satisfactory depth, hardness, and resistance to spelling. have fine grainedcores that possess satisfactory properties, and may be substituted, at least for many purposes, for the steels containing nickel and chromium that have been used previously for making carburized articles.

A further object is to provide carburized steel articles of high case hardness and whose cores are fine grained, which may be substituted for Bearing Company, Canton. Ohio,

purposes where steels containing nickel and chromium were used previously, and which exhibit longer life than articles made from those steels.

Other objects will be understood from the following specification.

The graphitic steels made in accordance with my Patent No. 2,087,764 have not been used for the making of carburized articles because their carbon content has been too high for that purpose. A graphitie steel, typical 0! that patent, which has been used extensively contains about 1.5 per cent of carbon, an average of 0.8 per cent of silicon, and about 0.25 per cent or molybdenum. I have now discovered that by greatly reducing the carbon content of such a steel while substantially increasing the contents of molybdenum and silicon, and by treating the steel in accordance with known graphitie steel practice, there are provided carburizing steels which can be carburized in accordance with existing standardized commercial practices to produce case hardened articles that not only can be substituted for those previously made from steels containing nickel or chromium, but also embody the advantages of graphitic steels.

This goes counter to the beliefs of those familiar with graphitic steels and carburizing steels. For example, the normal practice in the graphitic steel art is to reduce the content of graphitizing agent, silicon and molybdenum in this instance, as the carbon content is decreased, and normally it would be expected that when the carbon is reduced to the range necessary tor a carburizing steel, the silicon and molybdenum would have to be reduced greatly below that required for a graphitic steel containing, for example, 1.5 per cent of carbon.

In the present invention, however, the silicon, and preferably molydenum also, are increased as compared with the typical steel described as being made under my foregoing patent. Also, the belief in the carburizing art has been that silicon is detrimental because it might cause graphitisation or the case, and accordingly the S. A. E. speciflcations for the carburizing steels have set the maximium amount of silicon at 0.3 per cent. Contrary to those beliefs, I have now found that the graphitic steels to which this application is directed are desirable for carburizing purposes and are productive of carburized articles that possess a service life at least as good, and on the average substantially better, than that of articles made irom the S. A. E. carburiaing steels.

More particularly, the steels provided by this invention contain from about 0.1 to about 0.3 per cent of carbon, about 0.25 to 0.7 or 0.75 per cent of molybdenum, and silicon in an amount sufficient, in cooperation with the molybdenum present to effect graphitization upon heat treatment of the article. The silicon should be at least about 0.8 per cent, however, within the foregoing ranges I now prefer that the steels shall contain from about 0.15 to 0.25 per cent of carbon, about 0.4 to 0.6per cent of molybdenum, and about 1 to 1.25 per cent of silicon. The manganese content of the steels may vary from about 0.3 to about 0.8 per cent, but desirably is within about 0.4 to 0.7 per cent, and for most purposes the steels should contain a maximum of about 0.035 per cent each of phosphorus and sulfur.

The remainder of the steels is iron together with impurities in amounts customarily encountered in such steels, but it will be understood that other alloying elements may be present provided that they do not detrimentally affect the properties which characterize the present invention, for which reason the remainder of the steels may be said to be effectively iron inasmuch as such elements do not alter the essential character of products made in accordance with this invention. For example, nickel and chromium are unnecessary for the purposes of the present invention, but small amounts of them, such as may find their way into the steels through the use of scrap, are not harmful.

The steels are made suitably in an electric furnace, and they are treated in accordance with ordinary graphitic steel practice. That is, they are cast and hot worked in such manner that substantially all of the carbon remains in the combined form. The hot working is conducted to shape the articles, after which they are normalized and annealed to precipitate a portion of the carbon as graphitic carbon. After sizing, if necessary, they are carburized. For the shaping they may be hot worked, as by rolling or forging, and the steels forge satisfactorily at temperaturesbetween 2050 to 2200 F., which is higher than the previously produced graphitic steels are customarily worked. Normalizing is conducted by heating the shaped articles above the critical range and cooling in air, which results in a martensitic structure. Annealing may be accomplished by heating into or above the critical range according to known practices.

An advantage of these steels is that they may be carburized by either pack hardening or gas hardening practices such as have been used in the trade. After carburizing they may be heat treated, if need be, for example, to refine the grain and harden the case, and the hardened articles may then be tempered, if desired. To these ends the carburized articles may be heated to, for example 1550 F., quenched in oil, and tempered at, for example, 350 F.

Examination of carburized articles made in accordance with this invention has shown that the cores are sufiiciently fine grained for these purposes, being, for example, of average No. 7 or 8 grain size. The cases are satisfactorily deep and can be hardened as high as 66 Rockwell C, which is several points higher than is ordinarily had in commercial practice with S. A. E. carburizing steels. The cores are hardened rather uniformly to about 23 to 25 Rockwell C, and their fractures are of the type considered good for these purposes. Chemical analysis shows about 0.07 to 0.09 per cent of graphitic carbon, but ordinarily it is dispersed so finely as to be undetectable under the metallographic microscope.

The invention may be described further with reference to a 3500 pound heat of steel made and treated in accordance with the invention. It contained 0.19 per cent of carbon, 0.68 per cent of manganese, 0.6 per cent of molybdenum, 0.014 per cent of phosphorus, 1.42 per cent of silicon, and 0.013 per cent of sulfur, together with 0.17 per cent of chromium and 0.13 percent of nickel representing residual impurities contained in 10 scrap used in making the heat. The heat was cast into 11-inch square ingots which were forged at a temperature between 2250 and 2275 F. into about 6-inch blooms that were finally hammer forged into round bars 1% inches in diameter.

5 Samples of the forged bars were normalized from 1750 F., and examination of them showed a martensitic structure containing some ferrite and the following mechanical properties:

Yield point Ultimatestr., Elong., Red. ares, Hardness p. s. l. p. s. i. percent percent Brinell 25 Other samples of the bars were annealed in accordance with production practice used with S. A. E. 52,100, and chemical analysis showed that this resulted in the development of 0.07 per cent of graphitic carbon and a microscopic structure free from banding and comprising pearlite at the ferrite grain boundaries. The surface hardness of the annealed samples was 156 Brinell.

Other normalized samples were gas carburized at 1700 F. during 52%; hours, followed by direct oil quenching. Different samples were then reheated and quenched in oil, followed by tempering for 1 hour at 350 F. The core and case hardnesses produced were as follows:

In each instance the case structure was slightly coarse martensite while the core was composed of ferrite and martensite. Substantially similar results are obtained by pack hardening at 1650 to 1675 F. and quenching from the pot.

Hardenability was determined by the standard Jominy method using L-bar specimens. The data were plotted against similar data obtained with a typical production heat of S. A. E. 4620, which showed that the core hardness of the steel in accordance with this invention is somewhat less than that of S. A. E. 4620, which is advantageous because it represents improved ductility. These tests indicated that the maximum properties would be had by quenching from temperatures higher than those used, but that might result in distortion, the properties obtained are sufllcient for many purposes, and tests now to be reported showed that even though maximum properties are not developed by the foregoing treatments, actual parts made in accordance with the present invention exhibit better service life than similar parts made from such carburizing steels as S. A. E. 4620.

For the making of service life tests cups and cones for tapered roller bearings were made from the foregoing heat. These were assembled-with 75 rolls made from steel that has been used satisa,sa1,sss

ractorily. The parts were pack carburized tor 2% hours at 1710 F., given a diffusion heating for 1% hours at that temperature, quenched in oil, reheated to 1525 F., quenched in oil, and tempered at 360 F. Examination of the carburized cones showed that the case hardness was from 59 to 60.5 Rockwell C, and the core hardness was from 22.5 to 25 Rockwell C, with the case varying from 0.032 to 0.038 inch deep. The hearings were subjected to standard tests on a hydraulic machine used for testing purposes by one or the largest makers oi tapered roller bearinss in this country. Two tests were used; one involved operation or the bearing at 2619 R. P. M. under a 300 per cent combined load made up of 4910 lbs. radial stress and 1850 lbs. thrust, and the other involved only a radial load of 268 per cent. These tests indicated that the bearings were at least equal to those previously made from S. A. E. steels for this purpose.

According to the provisions of the patent statutes, I have explained the principle and mode of practicing my invention and have described what I now consider to represent its best embodiment. However, I desire to-have it understood that,- within the scope of the appended claims, the invention may be practiced otherwise than as specifically illustrated and described.

I claim:

1. A carburizing steel comprising about 0.1 to 0.3 per cent or carbon, about 0.3 to 0.8 per cent 01'.

3. Carburizing steel comprising about 0.15 to 0.25 per cent or carbon, about 0.3 to 0.8 per cent of manganese, about 0.4 to 0.6 per cent of molybdenum, about 1 to 1.25 per cent of silicon, and the remainder effectively iron, and being graphitizable by annealing to convert a portion or the carbon to graphitic carbon dispersed finely throush the structure.

4. Steel according to claim 3, containing about 0.4 to 0.7 per cent of manganese, and not over about 0.035 per cent each of phosphorus and suliur.

5. As a new article of manufacture, a carburized article formed from hot worked and graphitized steel comprising about 0.1 to 0.3 per cent of carbon, about 0.3 to 0.8 per cent of manganese,

about 0.2 to 0.75 per cent of molybdenum, silicon in an amount sui'ficlent to cause graphitization in manganese, about 0.2 to 0.75 per cent of molybdenum, silicon in an amount suillcient to cause graphitization in the presence of said molybdenum, and the remainder eflectively iron, and being graplutizable by annealing to converts portion of the carbon to graphitic carbon dispersed finely through the structure.

2. A carburizing steel comprising about 0.1 to 0.3 per cent of carbon, about 0.3 to 0.8 per cent of manganese, about 0.2 to 0.75 per cent of molybdenum, silicon at least about 0.8 p r cent and in an amount sumclent to cause graphitization in the presence or said molybdenum, not over about 0.035 per cent each of phosphorus and sulfur, and the remainder effectively iron, and being graphitizable by annealing to convert a portion oi the carbon to graphitic carbon dispersed finely through the structure.

the presence of said molybdenum, and the remainder efiectively iron, and containing a small portion of the carbon as graphitic carbon dispersed finely through the structure.

6. As a new article of manufacture, a carburized article formed from hot worked and graphitized steel comprising about 0.1 to 0.3 per centbof carbon, about 0.3 to 0.8 per cent of manganese, about 0.2 to 0.75 per cent or molybdenum, silicon at least about 0.8 per cent and in an amount sutflcient to cause graphitization in the presence of said molybdenum, not over about 0.035 per cent each of phosphorus and sulfur, and the remainder eilectively iron, and containing a small portion of the carbon as graphitic carbon dispersed finely through the structure.

7. As a new article of manufacture, a carburized article formed from hot worked and graphitized steel comprising about 0.15 to 0.25 per cent or carbon, about 0.3 to 0.8 per cent of manganese, about 0.4 to 0.6 per cent of molybdenum, about 1 to 1.25

40 per cent of silicon, and the remainder eflectively iron, and containing a small portion of the carbon as graphitlc carbon dispersed finely through the structure.

8. An article according to claim 7, said steel con- 46 taining about 0.4 to 0.7 per cent of manganese, and

not over about 0.035 per cent each 01' phosphorus and sulfur.

FREDERICK R. BON'I'E. 

